I called my water company and was told that the water in my town has a GH of 100 ppm. The lady said it was considered "moderately hard (as the moderately hard range is from 60 ppm to 120 ppm)".

I swapped out my light fixture for the T5 NO on Sunday when I did my water change and then added 6 ml (instead of 5 ml) of Flourish on Monday.

With the light change, do you think I should just stick with dosing the Flourish (6 ml) once a week for now or go ahead and increase it to twice a week? How long do you think it would take before I see any improvement?

I called my water company and was told that the water in my town has a GH of 100 ppm. The lady said it was considered "moderately hard (as the moderately hard range is from 60 ppm to 120 ppm)".

I swapped out my light fixture for the T5 NO on Sunday when I did my water change and then added 6 ml (instead of 5 ml) of Flourish on Monday.

With the light change, do you think I should just stick with dosing the Flourish (6 ml) once a week for now or go ahead and increase it to twice a week? How long do you think it would take before I see any improvement?

100pm equals to 5.6gh that's more on the soft side but should still be fine for your Vals. The reason Vals need more of hard water and lots of plants for that matter is there are minerals in the water they need and and the softer the water less there is in the water for them. I would just dose once a week for now. Give it a couple of weeks and see if there are any improvements. If you don't see any then you can try 2x but just watch out for algae out breaks. Hoping Byron see this thread as he has much more experience in this matter then I and explain much better then I can but for now these are things I would do.

The problem with the Vallisneria is likely the water softness. And as Boredomb said, 100ppm equates to 5.5 dGH and that is soft water. I add Equilibrium to get my zero-hardness tap water up to 5 to 6 dGH. My Vallisneria does not do well. Moss loves this, I am frequently pulling out rafts of it.

As for plants doing better in one tank and not another, this is likely due to varying biological systems. No two aquaria will be exactly the same with respect to water chemistry/biology, no matter how much we treat them the same with light, fertilizers, etc. I see this too, with my 7 tanks. Even moving a plant like floating Water Sprite from one tank to another will cause it to decline for a few weeks due to something different. And my tanks are about as close to being equal as one could expect. But nature still rules the day.

I agree with the suggested plan of action, to increase Flourish Comprehensive to twice weekly (the full dose each time, which is 2.5ml (1/2 teaspoon) per 30g or 5ml (1 teaspoon) per 60g, whichever, and change the light. With T5 HO tubes, you are setting up a much higher balance with nutrients, and plants will always grow up to the limiting factor, which here was nutrients. Lessening the light intensity will result in a lower balance, that should occur with the increased Flourish.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]